1. Field of the Invention
The present invention relates to a technology for detecting the state of a recording medium.
2. Description of the Related Art
Conventionally, there are image forming apparatuses such as copying machines, printers, and so forth, which have sensors to detect the state of recording media, inside of the image forming apparatus. These apparatuses automatically detect the state of a recording medium, and control transfer conditions (e.g., transfer voltage, conveyance speed of the recording medium at the time of transfer) and fixing conditions (e.g., fixing temperature, conveyance speed of the recording medium at the time of fixing), according to the detection results.
One example of a state of the recording medium to be detected is the moisture included in the recording medium. Different moisture amounts included in the recording medium changes the resistance value and heat capacity of the recording medium, so image quality may deteriorate of images are recorded on recording media under the same transfer conditions and fixing conditions. Accordingly, these apparatuses detect the amount of moisture included in the recording medium, and control the transfer conditions and fixing conditions according to the results of detection.
Japanese Patent Laid-Open No. 2008-145514 describes an image forming apparatus where a lever to detect the thickness of the recording medium is provided in a conveyance path. When the recording medium is conveyed, the lever is pressed upwards by an amount equivalent to the thickness of the recording medium, and the thickness of the recording medium can be detected by the amount of displacement of the lever. This image forming apparatus detects the amount of moisture included in the recording medium by comparing the thickness of the recording medium before passing through a fixing unit and after having passed through the fixing unit. The transfer conditions and the like are controlled according to the moisture amount detection results, thereby improving image quality.
However, the configuration described in Japanese Patent Laid-Open No. 2008-145514 is a configuration to detect thickness by the lever coming into direct contact with the recording medium, there are cases where the precision of thickness detection, and accordingly the precision of moisture amount detection, deteriorates due to the effects of flapping of the recording medium being conveyed. Also, in a case where the recording medium is thin paper, change in the amount of moisture hardly changes the thickness at all, so accurately detecting the amount of moisture has been difficult. Accordingly, while the configuration described in Japanese Patent Laid-Open No. 2008-145514 could obtain moisture amount detection precision sufficient for satisfying the image quality desired at that time, there has been demand in recent years for improved moisture amount detection precision, to satisfy the image quality demanded nowadays.